Indoor air quality has become a topic of considerable importance. Occupants of buildings, such as office spaces, restaurants and the like, are aware of and concerned about health issues raised by indoor air quality. Moreover, indoor air quality and associated health issues have received much publicity and have even been the subject of legal action. Therefore, there is great interest in improving indoor air quality.
There are many sources of space contaminants. Humans and their activities release a wide assortment of organic and inorganic chemicals. Personal care products, photocopy machines and other office equipment release chemicals. Growth of fungal material can release spores into the air. Standing water can allow harmful bacteria to multiply. Office furniture, partitions, paint, floor coverings and cleaning materials release chemicals into the air. Cracks in below-grade walls and floors can allow radon gas to enter the building.
The outdoor air itself can be a source of unwanted contaminants. This is particularly true of buildings located in large, congested metropolitan areas, or in cases where the building's outdoor air intake is located near loading docks, garages or near the building exhaust.
Improvement or control of indoor air quality has been addressed by two methods. One method is filtration or other removal of indoor air contaminants. However, this method is expensive and may be of limited effect, particularly with respect to gaseous contaminants such as volatile organic compounds (VOCs). A second method of improving internal air quality is dilution of contaminants by introduction of fresh, outside air. This method is more feasible, both technically and economically, than filtration.
Technical standards have been written to specify minimum ventilation rates necessary to dilute indoor air contaminants and maintain acceptable indoor air quality. For example, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) has issued ASHRAE Standard 62-1989 entitled "Ventilation For Acceptable Indoor Air Quality." This standard specifies two procedures for maintaining acceptable indoor air quality. The Indoor Air Quality procedure specifies control of known indoor air contaminants, similar to the filtration method described above. The Ventilation Rate Procedure specifies introduction of outside air of specified quality at specified rates to dilute indoor air contaminants.
ASHRAE Standard 62-1989 specifies supply rates of acceptable outdoor air required for acceptable indoor air quality for a variety of occupied spaces, including commercial, institutional and residential facilities. The ventilation supply rates specified in the standard are a function of factors such as the type of environment and the number of occupants. The outdoor air requirements are specified in units of cubic feet per minute (CFM) per person and specify the minimum prescribed supply rate of acceptable outdoor air required.
Conformance with the ventilation rate procedure of ASHRAE Standard 62-1989 requires supply of outside air at a controlled rate. This, in turn, requires measurement of the ventilation air flow rate so that it can be regulated to the specified rate. Therefore, closed loop control of the ventilation system is required for operation in conjunction with heating and air conditioning units.
Large scale heating, ventilating and air conditioning systems incorporate very sophisticated air flow measurement devices for monitoring supply of outside air. One common technique for measuring air flow in such systems uses one or more pitot tubes. These pitot tubes sense velocity pressure of air as it passes around the tube. The corresponding air velocity is related to the velocity pressure in a known manner. For measuring air flow, including outdoor air flow, pitot tube stations may be installed at various locations including the outdoor air intake of the heating, ventilating and air conditioning system. However, pitot tube systems are relatively expensive. To be economically feasible, an air flow measurement system, such as a pitot tube system, must be very inexpensive relative to the cost of the overall heating, ventilating and air conditioning system. Also, physical configuration of outdoor air ductwork typically does not provide space for installing pitot tubes. Also, low velocities of outdoor air make pitot tubes less accurate.
Many facilities employ low cost, self-contained "rooftop" heating, ventilating and air conditioning ("HVAC") systems, so called because the self-contained unit is typically installed on the roof of the facility. The rooftop unit includes, generally within a single structure, an air conditioning unit, a heating unit, a fan, an outdoor air intake including an outdoor air damper and a system controller. Placing such a unit into operation merely requires supply of utility service and coupling to the facility's air ducts and thermostat controls. Such a rooftop unit typically has a relatively small capacity, for example, in the range of 5-15 tons and may be used in conjunction with small commercial buildings, such as offices and fast food restaurants and the like, having a size up to, for example, 50,000 square feet.
Most smaller size rooftop units are characterized as being constant volume systems. That is, the supply fan associated with the unit runs at a generally fixed speed and supplies a constant volume of supply air to the occupied space.
Rooftop units are very cost sensitive. Design, installation and operation of such units must be inexpensive. To provide market differentiation, manufacturers seek to include a variety of operational features in such units.
Previous rooftop HVAC units made no provision for supply of outdoor air at a regulated, specified rate. One known practice for supplying outdoor air to a rooftop unit involves setting a fixed minimum position for the outdoor air intake damper, such as 10% open. The damper has a range of settings or positions, characterized as being between fully or 100% open and fully closed or 0% open. A specification of a fixed minimum damper position, as in previous rooftop HVAC units, is intended to ensure adequate outdoor air flow regulation in all modes of operation. However, such techniques have been generally ineffective at providing acceptable indoor air quality because they did not relate the damper position to the actual CFM outdoor air flow as required by the standard. Moreover, such techniques have not worked well in conjunction with other system controls, such as economizer controls.
Accordingly, there is a need in the art for a method and apparatus for controlling introduction of outside air into a constant volume system for controlling ventilation of an inside space.